Magnetic disk substrate and magnetic disk

ABSTRACT

The present invention relates to a magnetic disk substrate formed of a thermoplastic resin and a magnetic disk comprising a magnetic recording layer formed on the magnetic disk substrate. In the magnetic disk substrate of the invention, the amount of gases generated therefrom upon held at 90° C. for 1 hour is 100 μg/cm 2  or less, so that when scanned by a floating head, errors on recording and reading of information are remarkably reduced. Thus, the magnetic disk substrate can provide a magnetic disk having improved scan capability. The present invention also provides a process for manufacturing such a magnetic disk.

TECHNICAL FIELD

[0001] The present invention relates to a magnetic disk substrate formedof a thermoplastic resin and a magnetic disk comprising a magnetic layer(magnetic recording layer) formed on the magnetic disk substrate. Thepresent invention is also concerned with a process for manufacturing amagnetic disk from thermoplastic resins.

BACKGROUND ART

[0002] Magnetic disks such as hard disks and removable hard disks (e.g.,floppy disks and MO disks) are now used as external storage forcomputers. A magnetic disk is a disk coated with a magnetic material onits surface, and data are written on or read from the disk using amagnetic head.

[0003] Commonly, a hard disk for computers comprises a hard aluminumsubstrate coated with a magnetic material on its surface. One or pluralsuch hard disks are received in a closed casing, wherein they arerotated at high speed. A magnetic head is slightly floated from thesurface of the hard disk for data write/read purposes.

[0004] Thus, the hard disk is usually designed to write and readinformation by means of a magnetic head located in the vicinity of thesurface of the hard disk and floated thereover. Such a magnetic head iscalled a “floating head”. The distance between the magnetic head and thesurface of the hard disk is very short, ordinarily of the order of a fewμm or less. In addition, the hard disk is rotated at high speed; as themagnetic head comes in collision with the surface of the hard disk byreason of entrance of dust, impacts from outside, sudden power failures,etc., data may vanish or the magnetic head may break down. As a matterof course, when there are projections, if minute, from the surface ofthe hard disk, the magnetic head may collide therewith, causing errors.Although depending on the size and number of projections from thesurface of the hard disk, the magnetic head may break down due toimpacts on collision.

[0005] On the other hand, the magnetic head is mounted on the floatingslider to write and read data on and from the hard disk while slightlyfloated over the surface of the hard disk. To reduce spacing losses ofthe magnetic head as much as possible upon writing and reading, thefloating quantity of the magnetic head (the distance from the magnetichead to the surface of the hard disk) should now be reduced to thegreatest extent practicable. In some systems known to date, the floatingquantity is dwindled from a conventional several μm down to the order of50 to 70 nm.

[0006] To eliminate minute projections on the surface of the hard disk,an aluminum substrate for hard disks must greatly be improved in termsof flatness and smoothness. For this reason, aluminum substrates are nowfabricated by lathing or pressure annealing an aluminum substrate blankto remove the undulation of the principal surface, then forming anelectroless plating layer on that principal surface, and finallypolishing the plated layer to a mirror-smooth state.

[0007] However, such an aluminum substrate processing method istroublesome and poor in productivity, making it difficult to cut down onproduction costs. In addition, it is impossible to provide such analuminum substrate with the projection-and-pit arrangement necessary forwhere servo marks are to be formed.

[0008] In recent years, it has thus been put forward to fabricatemagnetic disk substrates such as hard disk substrates usingthermoplastic resins. Thermoplastic resins can easily be configured intomoldings of any desired shape by melt processing such as injectionmolding. In addition, if the surface roughness of molds or stampers usedfor injection molding is previously reduced, it is then possible toobtain moldings excelling in smoothness.

[0009] Even by use of injection molding of thermoplastic resins,however, it is in effect very difficult to obtain magnetic disksubstrates having high reliability.

[0010] JP-A 04-170425 proposes forming a disk substrate with a moldingmaterial that is a hydrogenated product of a ring-opening polymer of anorbornene monomer, wherein the volatile component content is 0.3% byweight or lower and the content of foreign matter of 0.5 μm or greateris 1×10⁵ or less. This disk substrate is reduced in terms of voids andsilver streaks resulting from volatile components as well as the contentof foreign matter, and so is useful for an optical disk substrate. Voidsand silver streaks are responsible for errors upon reading of signals byan optical disk using laser beams; however, molding defects such asvoids and silver streaks can be eliminated by decreasing the content ofvolatile components, thereby correcting the optical disk for possibleerrors.

[0011] However, the results of investigations by the inventors havetaught that although the disk substrate set forth in the aforesaidpublication is useful for an optical disk substrate, a reliabilityproblem arises when it is used for the substrate of a magnetic disk thatoperates proximately to a magnetic head as is the case with a hard disk.

[0012] Possible reasons could be that (1) no full removal oflow-molecular components is achievable under drying conditions that thehydrogenated products of ring-opening polymers specified in thepublication are vacuum dried at 260° C., and (2) at the foreign mattercontent level of 6×10⁴ to 9×10⁴/g described in the publication, anycomplete prevention of minute projections from the surface of the disksubstrate is unachievable.

[0013] WO 98/08217 proposes a disk substrate comprising a resinous disksubstrate, characterized in that on the surface to be scanned by afloating head there is no projection having a height of 50 nm orgreater. This publication also diskloses a disk substrate fabricationmethod by injection molding, wherein a resin solution is filtered by afilter to prepare a resin in which the content of particles of 0.5 μm orgreater in particle diameter is 1×10⁴/g or less, and the resin isinjection molded to a disk substrate.

[0014] The magnetic disk manufactured using the disk substrate set forthin the aforesaid publication, because of having no minute projection onthe surface to be scanned by a floating head, is found to be much morereduced in terms of errors upon writing and reading of information thanconventional disks. However, the results of studies by the inventorshave revealed that the effect of this magnetic disk on prevention oferrors is still less than satisfactory, and that adhesion between thedisk substrate and the magnetic layer becomes insufficient.

DISCLOSURE OF THE INVENTION

[0015] It is an object of the present invention to provide a magneticdisk substrate formed of a thermoplastic resin, which can provide amagnetic disk that, when scanned by a floating head, is remarkablyreduced in terms of errors upon writing and reading of information, andthat is excellent in scan capability.

[0016] Another object of the present invention is to provide a magneticdisk comprising a magnetic recording layer formed on the surface of theaforesaid magnetic disk substrate, which is excellent in scancapability.

[0017] Yet another object of the present invention is to provide aprocess for manufacturing a magnetic disk substrate that has suchexcellent properties.

[0018] As a result of intensive studies made so as to attain theseobjects, the inventors have found out that the reason why a magneticdisk is not free from an error problem irrespective of elimination ofprojections of 50 nm or greater in height from the surface of a disksubstrate lies in a trace amount of gas generated from a magnetic disksubstrate after molding.

[0019] Given a magnetic disk substrate susceptible to generation of gas,the gas generated from the magnetic disk substrate adheres to andsolidifies at the portion of a floating head nearest to the surface ofthe a magnetic disk. Then, the resulting solid lump comes into contactwith the surface of the magnetic disk, and so the floating headvibrates, ending up with the occurrence of errors.

[0020] The gas generated from the magnetic disk substrate also causes adecrease in the adhesion between the magnetic disk substrate and amagnetic recording film. This in turn causes the magnetic recording filmto peel off upon collision of the surface of the magnetic disk with thefloating head.

[0021] With these facts in mind, the inventors tried to makemodifications to drying conditions, etc. for thorough removal oflow-molecular-weight components having a molecular weight of 1,000 orlower capable of turning to gases from thermoplastic resins used as theraw material for magnetic disk substrates. As a consequence, theinventors have arrived at a magnetic disk substrate which is formed of athermoplastic resin and generates gases in an amount of 100 μg/cm² orless as measured upon held at 90° C. for 1 hour. When a magnetic disksuch as a hard disk is manufactured using a magnetic disk substratecontrolled such that the amount of gases per unit area upon held at aconstant temperature for a constant time is limited to a specific orsmaller amount, the occurrence of errors can significantly be reducedeven upon scanning of the surface of the magnetic disk by a floatinghead located in proximity thereto. The present invention has beenaccomplished based on these findings.

[0022] Thus, the present invention provides a magnetic disk substrateformed of a thermoplastic resin, wherein the amount of gases generatedtherefrom upon held at 90° C. for 1 hour is 100 μg/cm² or less.

[0023] The present invention also provides a magnetic disk, whichcomprises a magnetic recording film formed on the aforesaid magneticdisk substrate.

[0024] Moreover, the present invention provides a process ofmanufacturing a magnetic disk substrate formed of a thermoplastic resin,characterized by comprising a series of steps of:

[0025] (1) using as the thermoplastic resin an alicyclicstructure-containing polymer resin, and heating an organic solventsolution of the alicyclic structure-containing polymer resin underreduced pressure to dry the polymer and remove therefromlow-molecular-weight materials having a molecular weight of 1,000 orlower, thereby preparing a pellet,

[0026] (2) drying the pellet by heating, depressurizing or heating underreduced pressure, and

[0027] (3) forming a dried pellet into a magnetic disk substrate,

[0028] thereby obtaining a magnetic disk substrate wherein the amount ofgases generated therefrom upon held at 90° C. for 1 hour is 100 μg/cm²or less.

BRIEF DESCRIPTION OF THE DRTAWING

[0029]FIG. 1 is illustrative in section of one exemplary layerarrangement of the magnetic disk according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0030] 1. Magnetic Disk Substrate

[0031] (I) Amount of Gases Generated

[0032] The present invention provides a magnetic disk substrate formedof a thermoplastic resin, wherein the amount of gases generated uponheld at 90° C. for 1 hour is 100 μg/cm² or less. The amount of gasesgenerated from the magnetic disk substrate is preferably 80 μg/cm² orless, and more preferably 60 μg/cm² or less. In most cases, the amountof gases generated can be reduced down to 50 μg/cm² or less, especially30 μg/cm² or less. When the amount of gases generated is 20 μg/cm² orless, particularly preferable results are obtainable in terms of thesmoothness of the substrate.

[0033] When a magnetic disk is manufactured by forming a magneticrecording film on the magnetic disk substrate in which the amount ofgases generated per unit area is in the aforesaid range, the magneticdisk can be substantially free from errors because of no defoliation ofthe magnetic recording film or no contact of a floating head with thesurface of the magnetic disk. As the amount of gases generated becomestoo large, the gases generated from the magnetic disk substrate mayadhere to and solidify at the end of the floating head, possiblyresulting in contact of the resulting solid with the surface of themagnetic disk upon scanning by the floating head. Too much gases alsocause a decrease in the adhesion between the magnetic disk substrate andthe magnetic recording film.

[0034] The amount of gases generated from the magnetic disk substratemay be measured by dynamic headspace gas chromatography massspectrometry (DHS-GC-MS method). When this method is used, the gasesgenerated from the magnetic disk substrate are collected by means of asolid adsorbent and concentrated, and then poured in a sample pouringport of a DHS-GC-MS device.

[0035] (II) Low-Molecular-Weight Components

[0036] The gases generated from the magnetic disk substrate are composedof various low-molecular-weight components contained in thethermoplastic resin. By way of example but not by way of limitation,such low-molecular-weight components include unreacted monomers,oligomers, low-molecular-weight polymers, resin-decomposed products,additives, decomposed products of additives, organic solvents and wateras well as their mixtures. Typical low-molecular-weight componentsinclude those having a molecular weight of 1,000 or lower and turning togases at a temperature of 90° C. under normal pressure.

[0037] More specifically, the low-molecular-weight components having amolecular weight of 1,000 or lower are exemplified by (i) unreactedmonomer components remaining upon resin preparation, (ii) oligomercomponents (e.g., those having a polymerization degree of 10 or less),(iii) low-molecular-weight polymer components having a molecular weightof 1,000 or lower, (iv) resin-decomposed products, (v) organic solventsused for resin synthesis, (vi) moisture trapped in the resin duringresin preparation or resin storage, and (vii) additives and theirdecomposition products.

[0038] Usually, the molecular weight of low-molecular-weight componentsis a weight-average molecular weight (Mw) determined in terms ofpolyisoprene as measured by gel permeation chromatography (GPC) usingcyclohexane as a solvent.

[0039] The low-molecular-weight components contained in thethermoplastic resin differ depending on the type of the thermoplasticresin (the type of monomer), the type of additives, the type of organicsolvents, etc. By way of illustration but not exclusively, thelow-molecular-weight components include alkanes such as methane, ethane,propane and pentane; alcohols such as ethanol, propanol and butanol;ethers; esters such as dioctyl phthalate; organic acids such as formicacid and acetic acid; organic halogenide compounds such as methylchloride and dichlorobenzene; silicones such as dimethylsiloxane;norbornene compounds and their oxides; cyclohexyl group-containingcompounds; aromatic hydrocarbon compounds such as benzene, toluene andxylene; and ketones such as acetone and methyl ethyl ketone.

[0040] The proportion of a low-molecular-weight component having amolecular weight of 1,000 or lower contained in the thermoplastic resinis preferably 2% by weight or less, more preferably 1% by weight byless. In most cases, extremely satisfactory results are obtainable byuse of thermoplastic resins containing 0.5% by weight or less oflow-molecular-weight components.

[0041] (III) Shape

[0042] The magnetic disk substrate of the present invention has a diskshape having a center hole therein. The magnetic disk substrate of thepresent invention has a diameter of usually 10 to 500 mm, preferably 30to 200 mm and a thickness of usually 0.1 to 50 mm, preferably 0.2 to 5mm.

[0043] (IV) Other Properties

[0044] In addition to the requirement that the amount of gases generatedupon held at 90° C. for 1 hour is 100 μg/cm² or less, the magnetic disksubstrate of the present invention preferably has another requirementthat on the surface of the disk substrate to be scanned by a floatinghead there is no projection of 50 nm or greater in height.

[0045] To this end, it is preferable to use a thermoplastic resincontrolled such that the number of particles having a particle diameterof 0.5 μm or greater is reduced down to 1×10⁴/g or less. Thus, when amagnetic disk substrate is formed using a thermoplastic resin controlledsuch that the number of particles having a particle diameter of 0.5 μmor greater is reduced down to 1×10⁴/g or less, it is possible to reducethe number of particles contained in the magnetic disk substrate with aparticle diameter of 0.5 μm or greater down to 1×10⁴/g or less, therebyobtaining a magnetic disk substrate that has not substantially anyprojection of 50 nm or greater in height.

[0046] The magnetic disk substrate of the present invention is not onlyimproved in terms of adhesion with a magnetic recording film formedthereon, but is also reduced in terms of pits on its surface to bescanned by a floating head, which may otherwise be responsible forerrors upon writing and/or reading of signals. The pits ascribable toerrors are a concave form of surface defects having a maximum width of 1μm or greater and a depth of 20 nm or greater as viewed from above. Assuch concave defects exist on the surface to be scanned, errors occurdue to changes in the distance between the floating head and the surfaceof the substrate.

[0047] The number of pits having a depth of 20 nm or greater can moreprecisely be measured after the magnetic recording film has been formedon the surface of the magnetic disk substrate. That is, the surface ofthe magnetic recording film of the magnetic disk is scanned by thefloating head, so that sites where errors occur can be examined by a biterror analyzer. Then, the sites where errors are found are analyzedunder a scanning probe microscope to count the number of pits having adepth of 20 nm or greater. The number of pits accounts for the number ofpits as counted all over the surface scanned by the floating head. Thenumber of pits found on the surface of the magnetic disk substrate witha depth of 20 nm or greater is preferably 20 or less, more preferably 15or less, and even more preferably 10 or less. According to theinvention, it is possible to reduce the number of pits having a depth of20 nm or greater down to 8 or less, and especially 5 or less.

[0048] 2. Thermoplastic Resin

[0049] (I) Type of Thermoplastic Resin

[0050] No specific limitation is imposed on the thermoplastic resin usedin the present invention so far as it can be formed into a magnetic disksubstrate. Exemplary thermoplastic resins are styrene resins such aspolystyrene, acrylonitrile-styrene copolymers andacrylonitrile-butadiene-styrene copolymers; acrylic resins such aspoly(methyl methacrylate) (PMMA); polyester resins such as poly(ethyleneterephthalate), poly(butylene terephthalate) and liquid crystalpolyesters; polycarbonate resins (PC); polyamide resins such as nylon;aromatic engineering resins such as poly(phenylene ether),poly(phenylene sulfide) and polyether ether ketone; olefin resins suchas poly(methyl-1-pentene), polypropylene and polyethylene; and alicyclicstructure-containing polymer resins such as norbornene polymers, vinylalicyclic hydrocarbon polymers, mono-cyclic olefin polymers and cyclicconjugated diene polymers.

[0051] Among these, preference is given to PMMA, PC and alicyclicstructure-containing polymer resins in consideration of the dimensionaccuracy of the magnetic disk substrate, and particular preference isgiven to the alicyclic structure-containing polymer resins in view ofheat resistance and low water absorption.

[0052] (II) Alicyclic Structure-Containing Polymer Resin

[0053] The alicyclic structure-containing polymer resin preferably usedin the present invention is a polymer that has an alicyclic structure inits main chain and/or its side chain. In consideration of mechanicalstrength and heat resistance, preference is given to polymer resinshaving an alicyclic structure in their main chains.

[0054] Exemplary alicyclic structures are cycloalkane structures andcycloalkene structures; however, the cycloalkane structures arepreferred in view of mechanical strength and heat resistance.

[0055] The ring may be either a single ring or a condensed ring. Thenumber of carbon atoms that form the ring is in the range of usually 4to 30, preferably 5 to 20 and more preferably 1 to 15 in view ofmechanical strength, heat resistance and moldability.

[0056] The proportion of the repetition unit having an alicyclicstructure in the alicyclic structure-containing polymer resin mayappropriately be determined depending on what purpose the resin is usedfor; however, that proportion is ordinarily 50% by weight or greater,preferably 70% by weight or greater, and more preferably 90% by weightor greater. That the proportion of the repetition unit having analicyclic structure is in the aforesaid range is preferred with thetransparency and heat resistance of the obtained magnetic disk substratein mind.

[0057] Exemplary alicyclic structure-containing polymer resins are (1) anorbornene polymer, (2) a single-ring cyclic olefin polymer, (3) acyclic conjugated diene polymer and (4) a vinyl alicyclic hydrocarbonpolymer.

[0058] Among these, the norbornene polymer and vinyl alicyclichydrocarbon polymer are preferred, and the norbornene polymer is morepreferred in view of heat resistance and mechanical strength.

[0059] For the norbornene polymer, for instance, such known polymers asset forth in JP-A's 03-14882 and 03-122137, etc. may be used. To be morespecific, (i) ring-opening polymers of norbornene monomers, (ii)hydrogenated products of ring-opening polymers of norbornene monomers,(iii) addition polymers of norbornene monomers and (iv) additioncopolymers of norbornene monomers and vinyl compounds are mentioned.Among these, preference is given to the hydrogenated products ofring-opening polymers of norbornene monomers, the addition polymers ofnorbornene monomers and the addition copolymers of norbornene monomersand vinyl compounds in consideration of heat resistance, lightresistance and weather resistance, and particular preference is given tothe hydrogenated products of ring-opening polymers of norbornenemonomers.

[0060] The norbornene monomers, for instance, include norbornene,dimethanooctahydronaphthalene, trimethanododecahydroanthracene and theirsubstituted products; dicyclopentadiene, 2,3-dihydrodicyclopentadiene,dimethanooctahydrobenzoindene, dimethanodecahydrobenzoindene,dimethanodecahydrofluorene and their substituted products. Thesubstituents, for instance, include hydrocarbon groups such as alkyl,alkylidene and aryl groups; cyano group, halogen group, alkoxycarbonylgroup, pyridyl group, carboxyl group, and hydroxyl group. The norbornenemonomers may be used alone or in combination of two or more.

[0061] The vinyl alicyclic hydrocarbon polymers, for instance, includepolymers of vinyl alicyclic hydrocarbon monomers such asvinylcyclohexene and vinylcycloheane and their hydrogenated products;hydrogenated products of homopolymers of vinyl aromatic monomers such asstyrene and α-methylstyrene (in this case, the aromatic ring moiety isalso hydrogenated); and hydrogenated product of copolymers of α-olefinsor conjugated dienes and vinyl aromatic monomers (in this case, thearomatic ring moiety is also hydrogenated). For the copolymers, randomcopolymers, pseudo-random copolymers, block copolymer, gradient blockcopolymers, etc. may be used without restraint.

[0062] (III) Molecular Weight & Glass Transition Temperature

[0063] The molecular weight of the thermoplastic resin used in thepresent invention is suitably selected as necessary for the endapplication intended. The weight-average molecular weight of thealicyclic structure-containing polymer resin is in the range of usually5,000 to 500,000, preferably 8,000 to 200,000, and more preferably10,000 to 100,000 in terms of polyisoprene as measured by gel permeationchromatography (GPC) using a cyclohexane solution (or a toluene solutionin the case where the polymer resin is not dissolved in the cyclohexanesolution). When the weight-average molecular weight is in the aforesaidrange, the magnetic disk substrate excellent in the mechanical strengthand moldability can be obtained.

[0064] The glass transition temperature, Tg, of the thermoplastic resinused in the present invention is suitably selected as necessary for theend application intended. However, in view of heat resistance, the glasstransition temperature is usually 50 to 300° C., preferably 60 to 200°C., and more preferably 70 to 150° C.

[0065] 3. Production Process of the Magnetic Disk Substrate

[0066] The magnetic disk substrate of the present invention may beobtained by molding of the thermoplastic resin. In general, injectionmolding is preferably chosen from molding processes.

[0067] (I) How to Remove Low-Molecular-Weight Components

[0068] To obtain the magnetic disk substrate in which the amount ofgases generated upon held at 90° C. for 1 hour is restricted to 100μg/cm² or less, it is preferable to use as the thermoplastic resin asynthesized thermoplastic resin from which the low-molecular-weightcomponents are removed as much as possible. However, removal of thelow-molecular-weight components is not easy, and so it is required tofigure out various contrivances.

[0069] More specifically, the thermoplastic resin after synthesis isprocessed by appropriate combinations of the following methods, and thenapplied to the fabrication of magnetic disk substrates.

[0070] (i) Method in which the solution of the thermoplastic resin in anorganic solvent is heated under reduced pressure, thereby drying thesolution and removing low-molecular-weight components from it.

[0071] According to this method, it is preferable to heat thepost-synthesis reaction solution of the thermoplastic resin underreduced pressure (a pressure that is lower than normal pressure),thereby evaporating off the low-molecular-weight components contained inthe solvent and thermoplastic resin. This method will hereinafter becalled the “direct drying method”.

[0072] (ii) Method in which additives having a relatively high molecularweight are used in reduced amounts as the additives used if desired, forinstance, antioxidants.

[0073] According to this method, it is preferable to add a small amountof an antioxidant having a molecular weight of 700 or higher to thepost-synthesis reaction solution of the thermoplastic resin, and thenheat the reaction solution under reduced pressure for drying and removalof the low-molecular-weight components contained in the thermoplasticresin. The addition of such an antioxidant enables the thermoplasticresin to be kept from decomposition even by heating at high temperature.

[0074] (iii) Method in which, before the thermoplastic resin is moldedinto a magnetic disk substrate, a molding blank material formed of thatthermoplastic resin (typically a pellet) is heated and/or depressurized,followed by drying.

[0075] In the direct drying method (i), the organic solvent solutionsuch as the reaction solution is heated at a temperature in the range ofusually 270 to 340° C., and preferably 275 to 330° C. At too low aheating temperature, there is a decrease in the rate of removal oflow-molecular-weight components and solvent residues from thethermoplastic resin. At too high a heating temperature, thethermoplastic resin is vulnerable to decomposition by heat.

[0076] The heating temperatures in the aforesaid range are generallyhigher than the temperatures at which thermoplastic resins for opticaldisk substrates are prepared. With increasing heating temperatures, theresins suffer from discolorations by heat, which are not preferable forthe thermoplastic resins for optical disc substrates, because ofresulting in a light transmittance drop. When those resins are used formagnetic disc substrates, however, such discolorations as occurring uponheating have no influences on errors whatsoever.

[0077] The reduced pressure used in the direct drying method is usually26.7 kPa or lower, preferably 13.4 kPa or lower, and more preferably 6.7kPa or lower.

[0078] When the direct drying method is used, drying may be carried outwhile the heating temperature and the reduced pressure are variedstepwise or continuously. In particular, it is preferable to vary theheating temperature and/or the reduced pressure in a two-step fashion.Two-step drying can easily be carried out by using two or more solventremovers that can be heated and operated under reduced pressure. For thesolvent removers, a scraper type thin-film evaporator and a centrifugalthin-film evaporator is preferably used.

[0079] A preferable two-step drying process comprises the first stepwherein the organic solvent solution such as the post-synthesis reactionsolution of the thermoplastic resin is heated at a temperature of 270 to340° C. under a pressure of 6.7 to 26.7 kPa thereby removing the organicsolvent and other low-molecular-weight materials, and the second stepwherein the resulting solution is heated at a temperature of 270 to 340°C. under a pressure of less than 6.7 kPa thereby removing the remaininglow-molecular-weight materials.

[0080] In view of prevention of decomposition of the thermoplastic resinin the direct drying method, it is preferable to heat and dry underreduced pressure the organic solvent solution such as the post-synthesisreaction solution of the thermoplastic resin after the antioxidanthaving a molecular weight of 700 or higher has been added thereto. Theantioxidant is added in an amount of usually 0.01 to 1 part by weight,preferably 0.02 to 0.8 part by weight, and more preferably 0.03 to 0.5part by weight per 100 parts by weight of the thermoplastic resin. Toomuch antioxidant is likely to turn by itself into low-molecular-weightcomponents or its decomposition products are likely to turn into suchcomponents, resulting in the generation of gases.

[0081] To reduce the number of foreign particles contained in thethermoplastic resin with a particle diameter of 0.5 μm or greater, it ispreferable to filter the organic solvent solution such as thepost-synthesis reaction solution of the thermoplastic resin, using afilter before it is heated and dried under reduced pressure. To be morespecific, the post-synthesis reaction solution of the thermoplasticresin is filtered in a multistage manner using a filter having a largeaperture size of the order of 1 to 5 μm, then a filter having anaperture size of 0.5 to 1 μm, and then a filter having an aperture sizeof approximately 0.2 μm. Alternatively, those methods may be combinedtogether in a suitable manner. Yet alternatively, the reaction solutionmay be filtered by sole use of a filter having adsorptive power due tozeta potential.

[0082] After filtration, the resin solution is heated under reducedpressure in such a closed system as to prevent entrance of foreignmatter from outside environments for removal of volatile components.Then, the resin is palletized in an environment of high cleanness suchas in a clean room. The cleanness is placed under such strict control asto achieve a class of about 1,000 or lower, and preferably a class ofabout 100 or lower.

[0083] In view of removal of the low-molecular-weight component andreductions in the amount of gases generated, the method (iii) ispreferred, wherein a molding blank material such as a pellet comprisingthe thermoplastic resin is dried prior to molding. To dry the pellet,drying is carried out while the pellet is retained at a temperaturelower than the glass transition temperature, Tg, of the thermoplasticresin under a reduced pressure of 26.7 kPa or lower for 0.5 hour orlonger. The heating temperature for the pellet is in the range ofpreferably (Tg-100° C.) to (Tg-2° C.), and more preferably (Tg-50° C.)to (Tg-5° C.), and the reduced pressure is preferably 13.4 kPa or lower,and more preferably 6.7 kPa or lower. The pellet is preferably dried ina clean room having a class of cleanness of approximately 1,000.

[0084] By the aforesaid method, it is possible to obtain a pellet inwhich the content of low-molecular-weight components having a molecularweight of 1,000 or less is significantly reduced. By combining togetherthe aforesaid respective methods in various manners, it is also possibleto obtain a pellet in which the number of particles having a particlediameter of 0.5 μm or greater is controlled up to 1×10⁴/g or smaller.

[0085] (II) Additives

[0086] The thermoplastic resin may contain various additives representedby antioxidants, parting agents, lubricants, weather stabilizers,coloring agents (dyes and pigments), antistatic agents, soft polymers,and resins other than the thermoplastic resin.

[0087] Some additives, which are likely to bleed on the surface of themagnetic disk substrate or contain large amounts of low-molecular-weightcomponents, are not preferred. It is preferable to use additives havinga high molecular weight in reduced amounts. The additives have amolecular weight of preferably 700 or higher, and more preferably 1,000or higher, and be used in an amount of usually 0.01 to 1.0 part byweight, preferably 0.02 to 0.8 part by weight, and more preferably 0.03to 0.5 part by weight per 100 parts by weight of the thermoplastic resinalthough varying with the types of the additives.

[0088] While the additives may be used for molding in admixture with themolding blank material such as the pellet, it is preferable that theyare added to the organic solvent solution such as the post-synthesisreaction solution of the thermoplastic resin for the purpose of removingthe low-molecular-weight component.

[0089] (III) Molding Processes

[0090] Usually but not exclusively, hot-melt molding processes andsolution casting processes are all usable to mold the thermoplasticresin into a magnetic disk substrate. For mass production, however, itis preferable to rely on the hot-melt molding processes represented byinjection molding, extrusion molding, and compression molding. In viewof the dimensional accuracy of the substrate, prevention of a warp inthe substrate, the surface precision of the substrate, etc., it isparticularly preferable to rely on injection molding. To mold thethermoplastic resin into a magnetic disk substrate by hot-melt molding,the molding is preferably carried out after the thermoplastic resin ispalletized.

[0091] The amount of gases generated from the magnetic disk substrate,too, affects the molding conditions for the magnetic disk substrate.Given a conventional molding machine for optical disk substrates, thereis no critical limitation on the type of the injection molding machineused. Specifically, injection molding is preferably carried out using amold to which a stamper for magnetic disks is attached.

[0092] Although varying with the type of resin, the injection moldingtemperature (resin temperature) is in the range of usually 200 to 400°C., preferably 250 to 390° C., and more preferably 300 to 380° C. whenthe alicyclic structure-containing polymer resin is used. As long as themolding temperature is in the aforesaid range, the surface roughness,transferability, mechanical properties, etc. of the substrate areexcellent, and the resin is kept from decomposition by shear force andheat so that the amount of gases generated from the magnetic disksubstrate remains reduced. The mold temperature is in the range ofusually 70 to 140° C., preferably 90 to 140° C., and more preferably 90to 130° C. As long as the mold temperature is in the aforesaid range,the transferability of the magnetic disk substrate is excellent.

[0093] The molding of the thermoplastic resin into a magnetic disksubstrate is preferably carried out in an environment having highcleanness such as in a clear room. The cleanness during molding ispreferably placed under such strict control as to achieve a class ofabout 1,000 or less, and preferably about 100 or less.

[0094] By using the thermoplastic resin with the aforesaid process, itis possible to obtain a magnetic disk substrate in which, upon held at90° C. in 1 hour, the amount of gases generated is limited to 100 μg/cm²or less. By molding into a magnetic disk substrate the thermoplasticresin in which the number of particles having a particle diameter of 0.5μm or greater is controlled to 1×10⁴/g or less, it is possible to reducethe number of particles contained in the magnetic disk substrate with aparticle diameter of 0.5 μm or greater down to 1×10⁴/g or less.

[0095] Preferably for the magnetic disk substrate of the invention, theamount of gases generated is not only significantly reduced but also thenumber of pits having a depth of 20 nm or greater as measured from thesurface is reduced. In addition, on the surface of the substrate thereis substantially no projection having a height of 50 nm or greater.

[0096] 4. Magnetic Disk

[0097] The magnetic disk of the present invention is used as a magneticrecording medium for storing data in an information processor like acomputer. For instance, the magnetic disk of the invention is used as ahard disk.

[0098] Hard disks have a recording density of usually 100 megabytes (MB)or greater, and recently developed hard disks have a recording densityof as large as 1 gigabyte (GB) or greater. A hard disk has a rotationspeed in the range of usually 3,000 to 8,000 rpm, and magnetic recordingsignals are sensed by a magnetic head that is located proximately to andfloated over the surface of a magnetic disk.

[0099] The magnetic disk comprises a magnetic recording film formed onthe magnetic disk substrate of the invention. The magnetic recordingfilm is generally composed of an underlying layer, a magnetic layer anda lubricant coating layer.

[0100] Given below are some illustrative examples of the magnetic disksubstrate of the invention and the magnetic disk constructed using thesame.

[0101] The magnetic disk substrate preferably has a diameter of 30 to200 mm and a thickness of 0.2 to 5 mm. A typical hard disk has a diskform having a diameter of 65 mm and a thickness of 1.2 mm. By way ofillustration and not by way of limitation, the magnetic disk substratehas a structure comprising a recording area-forming portion, a clamparea at the inner radius of the substrate and a landing area at theouter radius of the substrate.

[0102] The recording area-forming portion provides a recording area ofthe magnetic disk when formed of the magnetic disk substrate. Thisrecording area-forming portion is composed of a data area-formingportion at which there is formed an area with data actually recordedthereon and a servo mark-forming portion at which there are formed servomarks for controlling (tracking) addresses and their positions inrecording tracks.

[0103] On the magnetic disk substrate there is provided an underlyinglayer, on which a magnetic layer is formed. On the magnetic layer thereare provided a protective film and a lubricant coating layer. To be morespecific, the underlying layer formed of chromium (Cr), molybdenum (Mo)or the like (and having a thickness of about 100 nm on the average), themagnetic layer formed of CoCrPt, CoPt, CoPd, Co, Pt, Pd or the like (andhaving a thickness of about 60 nm on the average) and the protectivefilm formed of C, SiO₂ or the like (and having a thickness of about 18nm on the average) are successively formed on the magnetic disksubstrate by means of sputtering.

[0104] For instance, sputtering is carried out in an argon (Ar) gasatmosphere using an in-line static opposition type DC magnetronsputtering system and employing an alloy target for the magnetic layer.A lubricant (e.g., Fomblin Z-DOL available from Montecatini) may becoated on the protective film by dipping or other suitable coatingmeans.

[0105] One specific example of the magnetic disk layer arrangement isshown in FIG. 1. FIG. 1 is illustrative in section of one example of thelayer arrangement of the magnetic disk. A Cr underlying layer 2, aCoPtCr layer 3, a Cr intermediate layer 4, a CoPtCr layer 5 and a carbon(C) protective layer 6 are formed on a magnetic disk substrate 1 in thisorder. The magnetic recording film has a thickness of usually about 100nm or less.

EXAMPLES

[0106] The present invention is now explained more specifically but notexclusively with reference to inventive examples and comparativeexamples wherein, unless otherwise stated, the “parts” and “%” are givenon a weight basis. Set out below are various physical properties and howto measure them.

[0107] (1) Weight-Average Molecular Weight (Mw):

[0108] The weight-average molecular weight (Mw) was determined in termsof polyisoprene as measured by gel permeation chromatography (GPC) usingcyclohexane as a solvent.

[0109] (2) Rate of Hydrogenation (%):

[0110] The rate of hydrogenation of the main chain and aromatic ring ofa norbornene polymer was measured by ¹H-NMR.

[0111] (3) Glass Transition Temperature:

[0112] The glass transition temperature (Tg) was measured according toJIS-K7121.

[0113] (4) Molecular Weight of Low-Molecular-Weight Components:

[0114] The weight-average molecular weight of a norbornene polymer wasdetermined in terms of polyisoprene as measured by gel permeationchromatography (GPC) using cyclohexane as a solvent. Then, the contentsof low-molecular-weight components having a molecular weight of 1,000 orlower were calculated by the proportion (%) of (B) with respect to[(A)+(B)] where (A) is the polymer peak area on the obtained chart and(B) is the peak area for the components having a molecular weight of1,000 or lower.

[0115] (5) Amount of Gases Generated:

[0116] The amount of gases generated from the magnetic disk substratewas measured as follows. The magnetic disk was cut with a constantsurface area to prepare samples. Each sample was heated at 90° C. for 1hour in a stream of nitrogen gas with a purity of 99.999% or greater.The gases given out of the sample were collected and concentrated by asolid adsorbent (an active charcoal tube), and the amount of theadsorbed gases was measured by dynamic headspace gas chromatography massspectrometry (DHS-GC-MS).

[0117] (6) Number of Pits on the Substrate:

[0118] The number of pits on the magnetic disk substrate was counted asfollows. After a magnetic recording film was formed on the magnetic disksubstrate to obtain a magnetic disk, the sites where errors occurred allover the surface of the magnetic disk scanned by a magnetic head wereexamined with a bit error analyzer (Bit Analyzer 622 manufactured bySyntheSye Research). The sites where errors occurred were each analyzedunder a scanning probe microscope (NanoscopeIIIa manufactured by DigitalInstrument) to count the number of pits having a depth of 20 nm orgreater.

[0119] (7) Rate of Unpeeled Recording Film:

[0120] After a magnetic recording film was formed on the surface of themagnetic disk substrate, a mending tape of 5 mm×5 mm size was applied onten sites of the magnetic recording film and then peeled off to measurethe area of unpeeled film sites. The rate of the unpeeled film sites wascalculated from

Rate of unpeeled film site=(the area of unpeeled film sites/the wholearea of contact of the mending tape)×100

[0121] (8) Scan Capability:

[0122] An induction magnetic head having a gap length of 0.6 μm and atrack width of 3.5 μm was used for a write magnetic head, and an MR headhaving a gap length of 0.36 μm and a track width of 2.5 μm was used fora read magnetic head. With the magnetic head floated 70 nm away from themagnetic disk, signals were written to and read out of the magnetic diskto measure readability (S/N value). The capital A indicates that signalsof 14 MHz frequency at a peripheral speed of 7 m/sec. have an S/N valueof 25 dB or greater, and the capital B shows that those signals have anS/N value of less than 25 dB.

Example 1

[0123] I. Resin Synthesis

[0124] A norbornene monomer mixture of 90% of6-methyl-1,4:5,8-dimethano-1,4,4a,5,6,7,8,8a-octahydronaphthalene(hereinafter MTD for short) and 10% of 5-methyl-2-norbornene wassubjected to ring-opening polymerization thereby obtaining a norbornenering-opening polymer.

[0125] One hundred (100) parts of the obtained ring-opening polymer weredissolved in 400 parts of cyclohexane to obtain a solution, to which 5parts of a nickel-alumina catalyst (manufactured by Nikki Kagaku) wereadded as a hydrogenation catalyst. While the solution was stirred at ahydrogen pressure of 50 kg/cm², it was heated to a temperature of 200°C. Then, the solution was subjected to a four-hour reaction to obtain areaction solution containing a hydrogenated product of a ring-openingpolymer (hereinafter called the “hydrogenated polymer”). UsingRadiolight #500 as a filtration bed, the reaction solution was subjectedto filtration under a pressure of 2.5 kg/cm² (Funda Filter manufacturedby Ishikawajima-Harima Heavy Industries) for removal of thehydrogenation catalyst, thereby obtaining a colorless transparentreaction solution.

[0126] II. Processing of the Resin

[0127] An antioxidant(3,3′,3″,5,5′,5″-hexa-tert-butyl-a,a′,a″-(mesitylene-2,4,6-triyl)tri-p-cresol(molecular weight=775) was added to the aforesaid reaction solution at aproportion of 0.05 part per 100 parts of the hydrogenated polymer. Then,the reaction solution was filtered through a metal fiber filter havingan aperture size of 3 μm (manufactured by Nichidai), then through a zetaplus filter 30H having an aperture size of 0.5 to 1 μm (manufactured byCuno), and finally through a metal fiber filter having an aperture sizeof 0.2 μm (manufactured by Nichidai), thereby removing foreign matter.

[0128] The solution from which foreign matter had been removed was ridof the solvent cyclohexane and other low-molecular-weight componentsresponsible for the generation of gases, using a cylindricalcondensation dryer (Hitachi, Ltd.). In this way, the hydrogenatedpolymer was recovered. Removal of the solvent and low-molecular-weightcomponents was carried out in a two-step manner.

[0129] The cylindrical condensation drier was run under the followingoperating conditions:

[0130] the first step at a temperature of 280° C. and a pressure of 13.3kPa (100 Torr), and

[0131] the second step at a temperature of 280° C. and a pressure of 0.7kPa (5 Torr).

[0132] III. Pelletizing

[0133] In a clean room of class 1,000, the hydrogenated polymer was hotextruded through a die joined directly to the condensation dryer, thencooled with water, then pelletized by a pelletizer (OSP-2 manufacturedby Osada Seisakusho), and finally recovered.

[0134] The pellet was dissolved in toluene to prepare a 10% toluenesolution for gas chromatography analysis. As a result, the amount ofcyclohexane remaining in the pellet was found to be lower than the lowerlimit of detection.

[0135] The heat decomposition temperature of the hydrogenated polymer byTG/DTA analysis was 442° C. The molecular weight in terms of apolystyrene of the hydrogenated polymer by GPC analysis was 27,000 forthe number-average molecular weight (Mn) and 56,000 for theweight-average molecular weight (Mw). The glass transition temperature(Tg) of the hydrogenated polymer was 140° C. as measured by DSCanalysis. The rate of hydrogenation of the pellet was substantially 100%as measured by ¹H-NMR spectroscopy in the form of a bichloroformsolution.

[0136] The recovered pellet was stored in a stainless closed vesselpolished on its surface.

[0137] IV. Drying of the Pellet

[0138] The pellet stored in the stainless closed vessel was pre-driedunder a pressure of 5 kPa and a temperature of 100° C. for 4 hours in avacuum drier installed in a clean room having a cleanness of class1,000.

[0139] V. Molding of the Magnetic Disk Substrate

[0140] Within 30 minutes after the pellet was taken out of the drier,the pellet was injection molded at a resin temperature of 320° C. and amold temperature of 120° C. through an injection molding machine (DISK3manufactured by Sumitomo Heavy Industries, Ltd.) installed in the sameclean room, thereby forming a magnetic disk substrate of 65 mm indiameter and 1.2 mm in thickness. The mold system was held in anenvironment having a cleanness of class 100 while the mold was housedtherein with a nickel stamper having a smooth surface (having acenter-line average roughness of Ra=1 nm).

[0141] The center-line average roughness, Ra, of the thus obtainedmagnetic disk substrate was 1.0 to 1.1 nm much on the same level as thesurface roughness of the stamper. This magnetic disk was set in amagnetic disk system. While the rotation speed of the magnetic disksubstrate was controlled such that a floating head was caused to floatin the state of floating quantity of 50 nm, acceleration signals uponcollision were detected by a piezo element attached to a slider. As aresult, it was found that on the surface of the magnetic disk substratethere was no projection of 50 nm or greater in height.

[0142] One point five (1.5) grams of a sample cut out of the magneticdisk substrate were dissolved at a concentration of 1.5% in toluenerefined by filtration through a cartridge filter having an aperture sizeof 0.2 μm. Then, a light scattering type fine particle detector (KS-58manufactured by Ryon) was used to count the number of foreign particlescontained in the toluene solution with a particle diameter of 0.5 μm orgreater. As a result, the toluene solution was found to contain 2.3×10³such particles per gram.

[0143] VI. Formation of the Magnetic Recording Film

[0144] Using a disk sputtering system C-3010 (manufactured by Aneruba),a chromium (Cr) underlying layer, a layer composed of cobalt(Co)/platinum (Pt)/Cr (CoPtCr layer), a Cr intermediate layer, a CoPtCrlayer and a carbon protective film were successively formed on themagnetic disk substrate obtained as mentioned above, thereby forming amagnetic recording film. The total thickness of the magnetic recordingfilm was 90 nm.

[0145] The amount of gases generated from this magnetic disk substratewas found to be 10 μg/cm². The magnetic disk substrate was thenestimated about its physical properties and characteristics. The resultsare tabulated in Table 1.

Example 2

[0146] A magnetic disk substrate and a magnetic disk were prepared as inExample 1 with the exception that the running conditions for thecylindrical condensation drier were changed to:

[0147] the first step at 280° C. and a pressure of 13.3 kPa, and

[0148] the second step at 280° C. and a pressure of 0.0 kPa (vacuumstate).

[0149] On this magnetic disk substrate there was no projection of 50 nmor greater in height, and the amount of gases generated from themagnetic disk substrate was 2 μg/cm². The results are tabulated in Table1.

Example 3

[0150] A magnetic disk substrate and a magnetic disk were prepared as inExample 1 with the exception that the running conditions for thecylindrical condensation drier were changed to:

[0151] the first step at 270° C. and a pressure of 13.3 kPa, and

[0152] the second step at 270° C. and a pressure of 0.0 kPa (vacuumstate).

[0153] On this magnetic disk substrate there was no projection of 50 nmor greater in height, and the amount of gases generated from themagnetic disk substrate was 60 μg/cm². The results are tabulated inTable 1.

Comparative Example 1

[0154] A magnetic disk substrate and a magnetic disk were prepared as inExample 1 with the exception that the running conditions for thecylindrical condensation drier were changed to:

[0155] the first step at 260° C. and a pressure of 13.3 kPa, and

[0156] the second step at 265° C. and a pressure of 1.3 kPa (10 Torr);

[0157] octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate(molecular weight=531) was used as the antioxidant; and the pellet wasnot subjected to heating under reduced pressure. On this magnetic disksubstrate there was no projection of 50 nm or greater in height, but theamount of gases generated from the magnetic disk substrate was increasedto 150 μg/cm². The results are tabulated in Table 1. TABLE 1 Comp. Ex.1Ex.2 Ex.3 Ex.1 Drying Condition <Concentration Drier> 1st step 280° C./280° C./ 270° C./ 260° C./ 13.3 kPa 13.3 kPa 13.3 kPa 13.3 kPa 2nd step280° C./ 280° C./ 270° C./ 265° C./  0.7 kPa  0.0 kPa  0.0 kPa  1.3 kPa<Pellet> 100° C./ 100° C./ 100° C./ —   5 kPa/   5 kPa/   5 kPa/ — 4 hs4 hs 4 hs — Content of low- 0.5 0.2 1.0 2.6 molecular-weight component,wt% Number per g of particles having a particle diameter of 0.5 μm orgreater Resin 2.1 × 10³ 2.0 × 10³ 2.2 × 10³ 2.3 × 10³ Substrate 2.3 ×10³ 2.2 × 10³ 2.5 × 10³ 2.6 × 10³ Amount of gases 10 2 60 150 generatedfrom substrate upon held at 90° C. for 1 hour, μg/cm² Number of pitsfound on 3.2 1.7 7.2 60.8 the surface of substrate with a depth of 20 nmor greater Adhesion with magnetic 100 100 100 50 recording film(unpeeling rate, %) Scan capability A A A B

INDUSTRIAL APPLICABILITY

[0158] The present invention provides a magnetic disk substrate that isimproved in terms of adhesion with a magnetic recording film,substantially free from errors upon writing and reading of data andexcellent in scan capability and a magnetic disk obtained using thesame. The present invention also provides a process of manufacturing amagnetic disk substrate that has such excellent properties. The magneticdisk substrate and magnetic disk of the present invention are useful forhard disks that are external memory devices for computers, etc.

1. A magnetic disk substrate formed of a thermoplastic resin, whereinthe amount of gases generated therefrom upon held at 90° C. for 1 houris 100 μg/cm² or less.
 2. The magnetic disk substrate according to claim1, wherein the thermoplastic resin is a thermoplastic resin in which thecontent of low-molecular-weight components having a molecular weight of1,000 or lower is 2% by weight or less.
 3. The magnetic disk substrateaccording to claim 2, wherein the low-molecular-weight componentsinclude unreacted monomers, oligomers, low-molecular-weight polymers,resin-decomposed products, additives, decomposed products of additives,water and organic solvents or mixtures thereof.
 4. The magnetic disksubstrate according to claim 1, wherein the thermoplastic resin is athermoplastic resin in which the number of particles having a particlediameter of 0.5 μm or greater is controlled to 1×10⁴/g or less.
 5. Themagnetic disk substrate according to claim 1, wherein the thermoplasticresin is an alicyclic structure-containing polymer resin.
 6. Themagnetic disk substrate according to claim 5, wherein the alicyclicstructure-containing polymer resin is at least one thermoplastic resinselected from the group consisting of a norbornene polymer, a polymer ofa single-ring cyclic olefin, a cyclic conjugated diene polymer and avinyl alicyclic hydrocarbon polymer.
 7. The magnetic disk substrateaccording to claim 6, wherein the norbornene polymer is at least onethermoplastic norbornene polymer selected from the group consisting of aring-opening polymer of a norbornene monomer, a hydrogenated product ofa ring-opening polymer of a norbornene monomer, an addition polymer of anorbornene monomer and an addition copolymer of a norbornene monomer anda vinyl compound.
 8. The magnetic disk substrate according to claim 7,wherein the norbornene polymer is a hydrogenated product of aring-opening polymer of a norbornene monomer.
 9. The magnetic disksubstrate according to claim 5, wherein the alicyclicstructure-containing polymer resin has a weight-average molecular weightin the range of 10,000 to 100,000.
 10. The magnetic disk substrateaccording to claim 1, which is a disk having a diameter of 30 to 200 mmand a thickness of 0.2 to 5 mm.
 11. The magnetic disk substrateaccording to claim 1, wherein the number of particles contained thereinwith a particle diameter of 0.5 μm or greater is 1×10⁴/g or less. 12.The magnetic disk substrate according to claim 1, wherein the number ofpits having a depth of 20 nm or greater, found on the surface to bescanned by a floating head, is 20 or less.
 13. A magnetic disk whichcomprises a magnetic recording film formed on a magnetic disk substrateas recited in any one of claims 1 to
 12. 14. A process of manufacturinga magnetic disk substrate formed of a thermoplastic resin, comprising aseries of steps of: (1) using an alicyclic structure-containing polymerresin as the thermoplastic resin, and heating a solution of thealicyclic structure-containing polymer resin in an organic solvent underreduced pressure, thereby drying said polymer resin and removinglow-molecular-weight materials having a molecular weight of 1,000 orlower; (2) drying the obtained pellet by heating, depressurizing, orheating under reduced pressure; and (3) using the dried pellet to form amagnetic disk substrate; thereby obtaining a magnetic disk substratewhich the amount of gases generated therefrom upon held at 90° C. for 1hour is 100 μg/cm² or less.
 15. The process according to claim 14,wherein in the step (1), a reaction solution obtained upon synthesis ofthe alicyclic structure-containing polymer resin is used as the solutionof the alicyclic structure-containing polymer resin in an organicsolvent.
 16. The process according to claim 14, wherein in the step (1),as the solution of the alicyclic structure-containing polymer resin inan organic solvent, a solution obtained by adding an antioxidant havinga molecular weight of 700 or higher to said solution at a proportion of0.01 to 1 part by weight per 100 parts by weight of said alicyclicstructure-containing polymer resin is used.
 17. The process according toclaim 14, wherein in the step (1), the solution of the alicyclicstructure-containing polymer resin in an organic solvent is heated at atemperature of 270 to 340° C. under a pressure of 26.7 kPa or lower fordrying.
 18. The process according to claim 17, wherein in the step (1),the solution of the alicyclic structure-containing polymer resin in anorganic solvent is heated at two steps wherein the first step involvesheating at a temperature of 270 to 340° C. under a pressure of 6.7 to26.7 kPa thereby removing the organic solvent and otherlow-molecular-weight materials and the second step involves heating at atemperature of 270 to 340° C. under a pressure of lower than 6.7 kPathereby further removing low-molecular-weight material residues.
 19. Theprocess according to claim 14, wherein in the step (2), the pellet isheld at a temperature lower than the glass transition temperature, Tg,of the alicyclic structure-containing polymer resin under a pressure of26.7 kPa or lower for 0.5 hour or longer, so that the pellet is dried.20. The process according to claim 14, wherein in the step (3), thedried pellet is injection molded at a resin temperature of 200 to 400°C. and a mold temperature of 70 to 140° C. into a magnetic disksubstrate.